9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A is the first and still the only marketed 15-membered semi-synthetic macrolide antibiotic from the group of azalides [The Merck Index, 12th Ed. (1996), p. 157 (946)]. It has the formula 
The synthesis of 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A is described in U.S. Pat. No. 4,517,359. Its antibacterial spectrum (J. Antimicrob. Chemother., 1987, 19, 275), mode of action (Antimicrob. Ag. Chemother., 1987, 31, 1939) and pharmacology (J. Antimicrob. Chemother. 1993, 31, Suppl. E, 1-198) are well known.
9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A occurs in amorphous form, and in several different crystal forms characterized by different arrangements of the atoms in the crystal network. Most of the forms are crystalline, their crystal unit cells containing, in addition to 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A, different numbers of water molecules and/or solvent molecules (pseudopolymorphs).
Anhydrous amorphous 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A, having a melting point of 113-115° C., is described in U.S. Pat. No. 4,517,359. It may be obtained by evaporation of the solvent from a chloroform solution of crude 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A. It is not crystalline but rather an amorphous product, resembling a solid foaming mass. A pure laboratory scale product may be obtained, either by chromatography of the crude final product or by dissolution of pure crystalline 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A monohydrate or dihydrate in an organic solvent, followed by evaporation of the solvent. Pure amorphous anhydrous 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A may be thus obtained. This procedure is not suitable for large-scale manufacture.
The preparation of various amorphous, crystalline solvated and hydrated forms of 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A has been described in the patent literature. See, for example, U.S. Pat. No. 4,474,768; U.S. Pat. No. 6,245,903; EP 1 103 558; CN 1 093370; CN 1 161971; WO 99/58541; WO 00/32203; WO 01/00640; WO 02/09640; WO 02/10144; WO 02/15842; WO 02/10181 and WO 02/42315. Materials so produced have been subject to various disadvantages including lack of purity, instability, hygroscopicity, and the like. Non-hygroscopic 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A dihydrate was prepared as early as the mid-1980's by neutralization of an acidic solution of 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A in an acetone-water mixture. Its crystal structure (single crystal) was evaluated upon recrystallization from ether, and was characterized by the orthorhombic space group P 212121. The unit cell parameters, namely crystal axes a=17.860 Å, b=16.889 Å and c=14.752 Å, and the angles between the crystal axes, α=β=γ=90°, were published in 1987 at the Meeting of Chemists of Croatia (Book of Abstracts, Meeting of Chemists of Croatia, Feb. 19-20, 1987, p. 29). Thereafter, its crystal structure and preparation were described in detail (J. Chem. Res. (S), 1988, 152, Ibid., miniprint 1988, 1239; received Jun. 4, 1987; Cambridge Crystallographic Data Base: GEGJAD).
Subsequently, in U.S. Pat. No. 6,268,489 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A dihydrate was described. That patent disclosed the preparation of the dihydrate by crystallization from tetrahydrofuran and hexane with the addition of water. The product thus formed is crystalline and can be obtained on a commercial scale in pure form. Its preparation is however subject to several disadvantages associated with the use of water-immiscible, toxic organic solvents and the necessity to carefully control the drying thereof.
Other techniques for preparing the dihydrate have been disclosed in the patent literature, e.g., in U.S. Pat. No. 5,869,629; EP 0 941 999; EP 1 103 558; HR P 921491; WO 01/49697; and WO 01/87912. Various of the procedures described involve the precipitation of the dihydrate by recrystallization from water-miscible solvents by the addition of water. The products formed by these and other processes described in the literature are however subject to a number of distinct disadvantages, ranging from the necessity to treat pharmaceutically pure 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A materials to the yield, purity and homogeneity of the products themselves. Indeed, products formed by various of the prior art techniques incorporate differing amounts of combined and adsorbed solvents and water, thus imparting inconsistent stability, purity, release and potency characteristics when incorporated in pharmaceutical formulations.
In HR patent application No. P20020231A monoclinic isostructural pseudopolymorphs of 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A are disclosed.
Now it has been surprisingly found out that a substantially pure amorphous 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A may also be prepared from crude 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A without using chromatography techniques, via new orthorhombic isostructural pseudopolymorphs of 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A intermediates.